1. Field of the Invention
The present invention relates to the field of spark-ignition engines fed with natural gas or with another power gas.
2. Description of the Prior Art
A problem inherent in spark-ignition engines lies in the wide range of natural gases available through the sales networks. In fact, great variations in the composition of the gases that are marketed have been observed. These gases can either correspond to the native gas or consist of a mixture of identified and known gases.
Therefore, although the composition of the gases available on the networks necessarily meets the standards of the countries where they are marketed, there are however great differences between two distributing points in one country; there are also differences between the distributions of different countries.
Furthermore, the composition of the gas delivered by a station can vary according to the period of the year; this is due to the supply variations of the stations.
Vehicles running on natural gas (NG) must however be supplied anywhere and anytime without any engine running problems.
These problems are already mentioned for example in U.S. Pat. No. 5,537,854 which describes a piezoelectric device allowing knock phenomena to be linked with the acoustic properties of the fuel (NG).
It is therefore necessary to know and/or to estimate the composition of the natural gas with which a given engine is supplied because the composition influences the running conditions of the engine, notably as regarding engine knock.
In fact, it is well-known that engine knock is correlated with the methane number (IM) of the gas, which itself depends on the composition of the gas, which directly influences the appearance of engine knock. Thus, if the methane number of a natural gas is known with sufficient precision, it will be possible, notably from engine maps, to deduce a corrective ignition advance value allowing the optimum ignition advance to be used in order to prevent knock phenomena or to allow optimum combustion adjustment.
A suitable engine tuning can thus be found.
Extensive research has already been carried out into natural gas characterization. Complex analysis systems have been described, for example in U.S. Pat. No. 5,333,591. According to this prior art, a device which analyzes of the composition of the natural gas, associated with a control unit, allows the engine to be tuned. This analysis is based here on the thermal conductivity of the gas.
It is also possible to use a chromatographic analysis to determine the composition of the natural gas. However, this solution is expensive and quite difficult to apply in a mobile and limited environment.
The present invention provides optimum engine running conditions regarding ignition advance, without risk of knocking. The invention improves the combustion while ensuring notably an optimum combustion velocity. The appearance of engine knock is advantageously prevented by decreasing the ignition advance as soon as required. Furthermore, the present invention provides control of ignition advance without the engine having to run in the knock zones, notably the initial knock zones.
The present invention provides, from at least one working point of the engine or reference point, a determination of the variation of a quantity linked with the variation of the flow of power gas entering the engine, these variations resulting from the change in the characteristics of the power gas, and from the variation of this quantity, determining the ignition advance correction to be applied at various working points of the engine.
The quantity linked with the variation of the flow of power gas can be the flow of gas itself It is possible, from this variation and from the air/power gas ratio of the mixture supplying the engine, to determine the stoichiometric air mass quantity to power gas mass quantity ratio corresponding to said power gas.
Finally, the ignition advance correction can be determined according to this stoichiometric ratio.
In particular, the present invention is a method of ignition advance correction of an internal-combustion engine according to the composition variation of the power gas, notably of the natural gas contained in a tank and supplying the engine. A methane number relative to the natural gas filling the tank is determined and an ignition advance correction value is determined from the number. A correlation is established according to the invention between the methane number described above, an air/fuel ratio R for a stoichiometric combustion and the variation of the flow of gas required to reach a reference working point of the engine after the composition variation of this gas.
More precisely, the following operations are carried out:
a) determining the gas flow variation required to reach a reference working point of the engine after the composition variation of this gas;
b) calculating, from this flow rate variation, a methane number (IM) relative to the natural gas filling said tank;
c) deducing an ignition advance correction value from said number IM; and
d) correcting the ignition advance.
A method according to the invention calculates a methane number relative to the natural gas filling the tank, then in correlating the methane number with an ignition advance correction value by means of a predetermined correspondence table, or in applying a given correction if the calculated number is below a predetermined value.
An air/fuel ratio R for stoichiometric combustion conditions depends on the composition of the natural gas in the tank of the vehicle. It has been found, within the scope of the present invention, that it is possible to establish a correlation between this ratio and the methane number described above.
Furthermore, the engine control unit can permanently quantify, according to the present invention, the effect of a variation of ratio R after a composition variation of the natural gas in the tank of a vehicle.
A suitable engine tuning can thus be found if the stoichiometric air/fuel ratio of the natural gas contained in the tank is permanently known.
More specifically, the methane number is thus calculated according to the air/fuel ratio (R) for a stoichiometric combustion. As already mentioned, ratio R depends on the chemical composition of the gas and it is therefore likely to change according to different tanks. More generally, this ratio changes as soon as the composition of the gas contained in the tank changes, i.e. when the composition of the fuel injected into the tank is different from that of the fuel previously present in the tank.
More precisely, ratio (R) is determined wherein (R(1)) is the ratio determined before filling the tank and (R(2)) is the ratio determined after filling the tank, and a flow of gas Q(1) before filling is determined and a flow of gas Q(2) after filing is determined, for the same working point of the engine. Ratio (R(1)) relative to the previous tankful and flow rate (Q(1)) are known because they are continuously stored in the computer memory. Significant variations in the gas compositions between two successive tankfuls thus correspond to notable variations between gas flows (Q(1)) and (Q(2)), thus between ratios (R(1)) and (R(2)). For the same reference working point of the engine (constant set air/fuel ratio and constant engine speed), it has been found that the stoichiometric Q air/Q fuel (gas) ratio corresponding to the gas contained in the tank is inversely proportional to the flow rate of this gas.
It is therefore possible to calculate the ratio R(2) from the difference of the inverse of the flow rates (1/Q(2))xe2x88x92(1/Q(1)) multiplied by a first constant Ki, a value to which ratio R(1), i.e. determined for the previous tankful, is added.
It has also been found in the present invention that it is possible to determine, in a surprisingly simple way, the methane number IM of the gas contained in the tank by multiplying ratio R(2) by a second constant K2 and by taking a third constant K3 away from the product. Values K2 and K3 can for example be determined according to the nature of all the gases likely to be used in a given place or country.